This invention relates to a chemically amplified positive resist composition for forming a contact hole pattern by the thermal flow process. While a method for forming a contact hole pattern using a chemically amplified positive resist composition comprising a polymer as the base resin involves the thermal flow step of heat treating the contact hole pattern for further reducing the size of contact holes, the invention relates to the resist composition to which a compound having functional groups capable of crosslinking with the polymer is added so that the size reduction by thermal flow becomes easy to control. The invention also relates to a method for forming a microsize contact hole pattern in the manufacture of VLSIs.
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. Deep-UV lithography is capable of achieving a minimum feature size of 0.3 xcexcm or less and, when a resist having low light absorption is used, can form patterns with sidewalls that are nearly perpendicular to the substrate.
Recently developed acid-catalyzed chemically amplified positive resists, such as those described in JP-B 2-27660, JP-A 63-27829, U.S. Pat. Nos. 4,491,628 and 5,310,619, utilize a high-intensity KrF or ArF excimer laser as the deep-UV light source. These resists, with their excellent properties such as high sensitivity, high resolution, and good dry etching resistance, are especially promising for deep-UV lithography.
Such chemically amplified positive resist compositions include two-component systems comprising a base resin and a photoacid generator, and three-component systems comprising a base resin, a photoacid generator, and a dissolution regulator having acid labile groups.
For example, JP-A 62-115440 describes a resist composition comprising poly-4-tert-butoxystyrene and a photoacid generator, and JP-A 3-223858 describes a similar two-component resist composition comprising a resin bearing tert-butoxy groups within the molecule, in combination with a photoacid generator. JP-A 4-211258 describes a two-component resist composition which is comprised of polyhydroxystyrene bearing methyl, isopropyl, tert-butyl, tetrahydropyranyl, and trimethylsilyl groups, together with a photoacid generator.
JP-A 6-100488 discloses a resist composition comprising a polydihydroxystyrene derivative, such as poly[3,4-bis(2-tetrahydropyranyloxy)styrene], poly[3,4-bis(tert-butoxycarbonyloxy)styrene] or poly[3,5-bis(2-tetrahydropyranyloxy)styrene], and a photoacid generator.
Improvement and development efforts have been continuously made on the base resin in resist compositions of this type. JP-A 10-207066 discloses a resist composition comprising a base resin which is crosslinked with crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages and a photoacid generator wherein the crosslinking groups are eliminated under the action of acid generated from the photoacid generator upon exposure, achieving a high contrast and high resolution.
Even when any base resin designed to enhance the resolving power is used in such chemically amplified positive resist compositions, it is yet difficult to reach a contact hole size of 0.20 xcexcm or less. There are available no resist compositions for forming a contact hole pattern satisfying the requirement of LSI devices of the next generation.
On the other hand, the known technology of forming a contact hole size of 0.20 xcexcm or less is to heat treat a contact hole pattern for causing the resist film to flow and reducing the contact hole size. This technology is known as thermal flow process. The use of the thermal flow process enables formation of a miniature contact hole size as fine as 0.10 xcexcm or 0.15 xcexcm.
In forming microsize contact holes by the thermal flow process, however, it is very difficult to control the heat treating temperature so as to provide a shrinkage matching with the desired contact hole size. That is, the thermal flow process has the drawback that even a slight variation of heating temperature brings about a substantial variation of contact hole size.
Referring to FIG. 1, there is illustrated in cross section a resist film 2 on a substrate 1, a contact hole 3 being formed through the resist film 2. The contact hole having undergone the thermal flow process has a profile as shown in FIG. 1, that is, a cross-sectional profile bowed at corners. The thermal flow process also has the problem that the profile of a contact hole is deteriorated.
An object of the invention is to provide a novel and improved chemical amplification type, positive working resist composition which has controllable process adaptability relative to the heat treating temperature when a microsize contact hole pattern is conventionally formed by the thermal flow process, and thus s has satisfactory practical utility. Another object is to provide a novel and improved method for forming a contact hole pattern.
It has been fount that when a contact hole pattern is formed by the thermal flow process using a chemically amplified positive resist composition comprising a compound containing at least two functional groups of the general formulas (1)-a to (1)-c in a molecule, the overall method is improved in process control and thus practically acceptable. 
Herein R1 to R4 are hydrogen or straight, branched or cyclic alkyl groups of 1 to 12 carbon atoms, R5 to R9 are independently straight, branched or cyclic alkyl groups of 1 to 12 carbon atoms, and a pair of R1 and R3, a pair of R4 and R5, a pair of R5 and R6, a pair of R7 and R8, a pair of R7 and R9 or a pair of R8 and R9, taken together, may form a ring. For brevity sake, formulas (1)-a to (1)-c are sometimes referred to as formula (1), hereinafter.
Specifically, making the investigations to be described below, the inventor has established the method of controlling the thermal flow process.
In the inventor""s experiment, a variety of base resins commonly used in conventional chemically amplified positive resist compositions were used to form resist films, which were provided with contact holes and subjected to the thermal flow process. The contact hole size was plotted relative to the heating temperature in a graph. It was found that the slope representing a change of contact hole size (to be referred to as a flow rate, hereinafter) was not so different among different base resins. Namely, changing the base resin skeleton gives no substantial difference in the flow rate. The flow rate remains substantially unchanged whether the base resin is a homopolymer or a copolymer and when the molecular weight or dispersity of the base resin is changed. This is also true when the acid labile group and other substituents are changed. The flow rate does not depend on the percent and type of substitution. Blending two or more distinct polymers brings little change of the flow rate. Through these investigations, it was found that only the flow initiation temperature, that is, the temperature at which the contact hole size becomes reduced changes with the base resin and depends on the glass transition temperature (Tg) of the base resin.
A summary of these findings can be illustrated in the graph of FIG. 2. In FIG. 2, curve I denotes a low molecular weight polymer, curve II denotes polymer A, curve III denotes polymer B, curve IV denotes a blend of polymer A and polymer B, curve V denotes a polymer having crosslinking groups, curve VI denotes a high molecular weight polymer, and curve VII denotes a polymer having a high Tg. The gradient of the curve represents the flow rate.
The flow rate can be numerically represented by a change of the contact hole size per degree centigrade of the heating temperature (unit: nm/xc2x0C.). While the base resin was changed among a variety of polymers, the flow rate did not substantially change. The change of contact hole size per degree centigrade was approximately 20 nm/xc2x0C. In the fabrication of LSI devices of the next generation targeting further miniaturization, the flow rate of 20 nm/xc2x0C. is difficult to control, inadequate to process adaptability, and by no means permissible.
Based on the above findings, the inventor made further investigations to find that when a contact hole pattern is formed by the thermal flow process using a chemically amplified positive resist composition comprising a compound containing at least two functional groups of the general formula (1) in a molecule, there are achieved a reduced flow rate, improved process control and practical utility.
By adding a compound containing at least two functional groups of the general formula (1) in a molecule to a chemically amplified positive resist composition, the flow rate in the thermal flow process, that is, the change of contact hole size per degree centigrade of heating temperature is improved as demonstrated in the graph of FIG. 3. In FIG. 3, curve A denotes a composition having the relevant compound added thereto and curve B denotes a control composition (free of the relevant compound).
When the compound containing at least two functional groups of the general formula (1) in a molecule is formulated together with a base resin in a chemically amplified positive resist composition, thermal crosslinking reaction can take place between the functional groups and the base resin. This thermal crosslinking reaction proceeds at the heat treating temperature during the thermal flow process, interfering with the rate at which the resist film is fluidized and thereby reducing the flow rate of contact holes. It is believed that the thermal crosslinking reaction of the compound containing at least two functional groups proceeds predominantly with phenolic hydroxyl groups on the base resin, but to some extent, with other sites on the base resin.
After a chemically amplified positive resist composition having the compound containing at least two functional groups of the general formula (1) in a molecule added thereto was used to form a resist film, which was provided with contact holes and-subjected to the thermal flow process, the resulting contact hole pattern configuration was observed. The contact hole pattern was improved in perpendicularity as compared with a control resist composition (without the compound containing at least two functional groups) yielding a contact hole pattern having rounded sidewalls at the end of thermal flow.
In summary, it has been found that the addition of the compound containing at least two functional groups of the general formula (1) is effective for reducing the flow rate associated with the thermal flow process of forming a microsize contact hole pattern and that the composition is effectively controllable and process adaptable in the fabrication of LSI devices of the next generation targeting further miniaturization.
Accordingly, the invention in a first aspect provides a chemically amplified positive resist composition for forming a contact hole pattern by the thermal flow process, comprising a compound containing at least two functional groups of the general formulas (1)-a to (1)-c in a molecule. 
Herein R1 to R4 are independently hydrogen or straight, branched or cyclic alkyl groups of 1 to 12 carbon atoms, R5 to R9 are independently straight, branched or cyclic alkyl groups of 1 to 12 carbon atoms, and a pair of R1 and R3, a pair of R4 and R5, a pair of R5 and R6, a pair of R7 and R8, a pair of R7 and R9 or a pair of R8 and R9, taken together, may form a ring.
In one preferred embodiment, the compound containing at least two functional groups is of the general formula (2). 
Herein Z is a functional group selected from the formulas (1)-a to (1)-c, the Z""s may be the same or different, k is a positive integer of 2 to 6, and X is a k-valent organic group of 2 to 20 carbon atoms.
Typically the compound of the general formula (2) is present in an amount of 0.1 to 5% by mass of the overall resist composition.
In a second aspect, the invention provides a chemically amplified positive resist composition for forming a contact hole pattern by the thermal flow process, comprising (A) an organic solvent, (B) a base resin in the form of a polymer having acid labile groups, (C) a photoacid generator, (D) a basic compound, and (E) a compound containing at least two functional groups of the general formulas (1)-a to (1)-c in a molecule, as set forth above.
In one preferred embodiment, the base resin (B) is a polymer comprising recurring units of the following general formula (3) in which some of the hydrogen atoms of the phenolic hydroxyl groups are partially replaced by acid labile groups of at least one type, and some of the hydrogen atoms of the remaining phenolic hydroxyl groups are optionally eliminated for crosslinkage within a molecule and/or between molecules with crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages, the total of the acid labile groups and the crosslinking groups being more than 0 mol % to 80 mol % of the entire hydrogen atoms of phenolic hydroxyl groups in the formula (3). The polymer has a weight average molecular weight of 1,000 to 500,000. 
Herein R10 is hydrogen or methyl, R11 is hydrogen or a methyl, phenyl or cyano group, R12 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms, R13 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 10 carbon atoms, x is 0 or a positive integer of up to 5, y and z are positive integers satisfying y+zxe2x89xa65, m and p are 0 or positive numbers, n is a positive number, satisfying 0xe2x89xa6m/(m+n+p) xe2x89xa60.8, 0 less than n/(m+n+p)xe2x89xa61, and 0xe2x89xa6p/(m+n+p)xe2x89xa60.8.
In a further preferred embodiment, the base resin (B) is a polymer represented by the following general formula (4), that is a polymer comprising recurring units of the general formula (3) in which some of the hydrogen atoms of the phenolic hydroxyl groups are partially replaced by acid labile groups of at least one type, and some of the hydrogen atoms of the remaining phenolic hydroxyl groups are optionally eliminated for crosslinkage within a molecule and/or between molecules with crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages, the total of the acid labile groups and the crosslinking groups being more than 0 mol % to 80 mol % of the entire hydrogen atoms of phenolic hydroxyl groups in formula (3). The polymer has a weight average molecular weight of 1,000 to 500,000. 
Herein
R10 is hydrogen or methyl, R11 is hydrogen or a methyl, phenyl or cyano group, R12 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms, R13 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 10 carbon atoms, R14 is an acid labile group of at least one type, R15, R16, R18 and R19 are independently hydrogen or straight, branched or cyclic alkyl groups of 1 to 6 carbon atoms, and R17 is a divalent or polyvalent hydrocarbon group of 1 to 10 carbon atoms which may be separated by an oxygen atom,
each said unit may be constructed of one type or at least two types,
x is 0 or a positive integer of up to 5, y and z are positive integers satisfying y+zxe2x89xa65, a, b and c are positive integers satisfying a+b+cxe2x89xa65, with the proviso that c is not equal to 0, d, e and f are 0 or positive integers satisfying d+e+fxe2x89xa64,
q, t and u are 0 or positive numbers, r and s are positive numbers, satisfying 0xe2x89xa6q/(q+r+s+t+u)xe2x89xa60.8, 0 less than s/(q+r+s+t+u)xe2x89xa60.8, 0xe2x89xa6t/(q+r+s+t+u)xe2x89xa60.8, 0xe2x89xa6u/(q+r+s+t+u)xe2x89xa60.8, 0 less than (r+s+t)/(q+r+s+t+u)xe2x89xa61, and 0 less than r/(q+r+s+t+u)xe2x89xa60.8.
In a further preferred embodiment, the base resin (B) is a polymer represented by the following general formula (5), that is a polymer comprising recurring units of the general formula (3) in which some of the hydrogen atoms of the phenolic hydroxyl groups are partially replaced by acid labile groups of at least one type, and some of the hydrogen atoms of the remaining phenolic hydroxyl groups are optionally eliminated for crosslinkage within a molecule and/or between molecules with crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages, the total of the acid labile groups and the crosslinking groups being more than 0 mol % to 80 mol % of the entire hydrogen atoms of phenolic hydroxyl groups in formula (3). The polymer has a weight average molecular weight of 1,000 to 500,000. 
Herein
R11, R12, R13, R14, R15, R16, R17, R18, R19, x, y, a, b, c, d, e, and f are as defined above,
R21 and R22 are independently hydrogen or straight, branched or cyclic alkyl groups of 1 to 8 carbon atoms, R23 is a monovalent hydrocarbon group of 1 to 18 carbon atoms which may have a hetero atom, a pair of R21 and R22, a pair of R21 and R23 or a pair of R22 and R23, taken together, may form a ring, each of R21, R22 and R23 is a straight or branched alkylene group of 1 to 8 carbon atom when they form a ring, R24 is a tertiary alkyl group of 4 to 20 carbon atoms,
g is 0 or a positive integer of 1 to 6, q, s1, s2, s3, t and u are 0 or positive numbers, r is a positive number, satisfying 0xe2x89xa6q/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0xe2x89xa6s1/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0xe2x89xa6s2/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0xe2x89xa6s3/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0 less than (s1+s2+s3)/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0xe2x89xa6t/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0xe2x89xa6u/(q+r+s1+s2+s3+t+u)xe2x89xa60.8, 0 less than (r+s1+s2+s3+t)/(q+r+s1+s2+s3+t+u)xe2x89xa61, and 0 less than r/(q+r+s1+s2+s3+t+u)xe2x89xa60.8.
In the preferred resist composition, component (C) is an onium salt and/or diazomethane derivative; and component (D) is an aliphatic amine.
In a third aspect, the invention provides a method for forming a contact hole pattern, comprising the steps of (i) applying the chemically amplified positive resist composition of any one of claims 1 to 9 onto a substrate to form a coating, (ii) heat treating the coating and exposing the coating to high energy radiation with a wavelength of up to 300 nm or electron beam through a photo-mask, (iii) optionally heat treating the exposed coating, and developing the coating with a developer, thereby forming a contact hole pattern, and (iv) further heat treating the contact hole pattern for reducing the size of contact holes.